How do we compare the image quality of two cameras? Do we have some tangible parameters to make such an assessment, and what are the main aspects of the image which contribute to image quality? Is image sharpness one of the criterion for image quality?

I'd guess the question is: if we say camera A takes better quality pictures than camera B, then what factors make up that assessment? Sharpness, color rendition, dynamic range, noise levels - lens and sensor attributes mainly I guess. The body is just a life-support system for the sensor. Not sure this is 100% a duplicate, but needs clarifying.
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MikeWApr 1 '13 at 18:24

Yeah, but with DSLRs, the lens throws a big confounding factor into the mix. The only reasonable way I can think of doing these tests is to get a third party fully manual prime lens with exceptional IQ (like the Zeiss stuff) and pair it with the different cameras.
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Chinmay KanchiApr 1 '13 at 18:52

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I'm not aware of any particular tangible, scientific measure for overall image quality. There are certain types of artifacts that are to be expected and if you know what you are looking for you can identify distortions that are caused by them, but ultimately, it's still a somewhat subjective measure at some level.

The general idea is to measure how closely the image captured corresponds to what was present in the scene. Sharpness can certainly be a big part of image quality measures, as can the dynamic range of the sensor (which can be measured more scientifically) and the color accuracy of the sensor (which can be compared to a dedicated colorometer or spectrometer. Other things like various types of lens distortion have known looks that can be examined based on how they look. How small of an area is impacted, how much are lines and gradients distorted, etc.

Really, anything that impacts the level of detail and accuracy of an image contributes towards the image quality and the amount we can objectively vs subjectively measure depends on the characteristic that is being examined. Also, how various factors should be balanced is really subjective and can differ from one type of image to another.

The real problem is, most (all?) of these things are also heavily influenced by the lens, making DSLR comparison across brands a difficult process.
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Chinmay KanchiApr 1 '13 at 17:16

@ChinmayKanchi - this is very true. While a camera body and sensor will act as a cap on potential image quality, the lens involved will have a far larger impact in the vast majority of cases. Dynamic range is dependent solely on the sensor, but just about everything else I mentioned is dependent in part or whole on the lens.
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AJ HendersonApr 1 '13 at 17:18

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Even with DR, the apparent DR of the scene is probably also influenced by the contrast of the overall image, which is heavily influenced by the lens (and indeed is tied to sharpness).
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Chinmay KanchiApr 1 '13 at 17:24